Conventionally known are circulating fluidized bed boilers as shown in References 1 and 2, JP 2005-274015A and JP 2004-132621A, respectively. FIG. 1 shows a circulating fluidized bed boiler of Reference 1 comprising a fluidized bed combustion furnace 1 for heating of particles (sand) through fluidized combustion by supply of fuel A into a fluidized bed of the particles fluidized through blowing-in of air, a separator 5 in the form of a cyclone for introduction of burnt gas 2 from a top of the furnace 1 and separation of the burnt gas into hot particles 3 and exhaust gas 4, a particle storage 7 for storage of the hot particles 3 separated in the separator 5 and introduced through a downcomer 5a, the stored particles 3 being circulatorily supplied via particle supply means 6 in the form of a so-called J- or L-valve type communicating pipe 6a to a lower portion of the fluidized bed combustion furnace 1, a heat transmission portion 8 as boiler for recovery of heat from the exhaust gas 4 and a bag filter 9 for removal of ash from the gas 4.
The particle storage 7 is supplied with air 14 from below by air supply means 10 to form a fluidized bed 11. The particle supply means 6 in FIG. 1 comprises the J- or L-valve type communicating pipe 6a with a lower end connected to the inside lower portion of the fluidized bed combustion furnace 1 and an upper end opened at 12 into the fluidized bed adjacent to a bottom of the particle storage 7, thus providing a backflow preventive structure preventing the fluid gas in the furnace 1 from flowing back into the separator 5. The communicating pipe 6a is provided with a movable flow rate controller 13 adjacent to the opening 12 to control a circulatory amount of particles to the fluidized bed combustion furnace 1.
In the fluidized bed combustion furnace 1 in FIG. 1, the particles are heated by fluidized combustion through supply of air and fuel A; burnt gas 2 from the furnace 1 is introduced into the separator 5 where it is separated into hot particles 3 and exhaust gas 4, the former being supplied to the particle storage 7. Then, the particles 3 in the particle storage 7 is sequentially taken out by a predetermined amount by the J- or L-valve type communicating pipe 6a to be circulatorily supplied to the fluidized bed combustion furnace 1 where the particles are heated again. In this connection, the circulatorily supplied amount of the particles 3 from the particle storage 7 to the fluidized bed combustion furnace 1 is controlled by the flow rate controller 13 provided adjacent to the opening 12 of the communicating pipe 6a. According to the construction with the particle storage 7 and the fluidized bed combustion furnace 1 connected together through the J- or L-valve type communicating pipe 6aj, the fluid gas in the fluidized bed combustion furnace 1 can be prevented from flowing back into the separator 5.
However, the circulatory amount of the particles 3 taken out through the communicating pipe 6a from the particle storage 7 into the fluidized bed combustion furnace 1 is relatively small, and cannot be controlled to be increased since the flow rate controller 13 serves only for throttling a flow passage in the communicating pipe 6a; thus, the circulatory amount of the particles 3 cannot be controlled over a larger control range. The flow rate controller 13, which has a movable portion required to moved within the communicating pipe 6a for control of the circulatory amount of the particles 3, requires countermeasure to high temperature and therefore is disadvantageously complicated in structure.
FIG. 2 shows a circulating fluidized bed boiler according to Reference 2 which is substantially identical in structure with that shown in FIG. 1, particles 3 from a separator 5 being introduced through a downcomer 5a′ into below a surface layer of a fluidized bed 11 in a particle storage 7, thus providing a backflow preventive structure for preventing fluid gas in a fluidized bed combustion furnace 1 from flowing back into the separator 5. The fluidized bed 11 in the particle storage 7 at the surface layer thereof is connected to the fluidized bed combustion furnace 1 at a lower position thereof through particle supply means 6 in the form of a slanted pipe 6b, the particles 3 in the surface layer of the fluidized bed 11 overflowing through an upper end of the slanted pipe 6b to be circulatorily supplied to the lower portion of the fluidized bed combustion furnace 1. In the system shown in FIG. 2, a supplied amount of air 14 to the particle storage 7 by air supply means 10 is controlled to vary in height the surface layer of the fluidized bed 11 (layer height), thus controlling the circulatory amount of the particles 3 from the particle storage 7 to the fluidized bed combustion furnace 1.
According to the system in FIG. 2, the supplied amount of air 14 to the particle storage 7 is controlled to vary in height the surface layer of the fluidized bed 11 to thereby control the circulatory amount of the particles 3 from the particle storage 7 to the fluidized bed combustion furnace 1, so that the circulatory amount of the particles 3 can be controlled easily and over a wider control range.
Recently, there has been proposed a circulating fluidized bed furnace so-called twin tower type gasification furnace and comprising a fluidized bed combustion furnace and a fluidized bed gasification furnace. The circulating fluidized bed furnace is disclosed for example in Reference 3 (JP 2005-41959A).
FIG. 3 shows the circulating fluidized bed furnace in Reference 3 comprising a fluidized bed combustion furnace 100 for heating of particles through combustion of char in a fluidized bed supplied with air, a separator 104 for introduction of burnt gas 101 from the furnace 100 and separation of the same into hot particles 102 and exhaust gas 103 and a fluidized bed gasification furnace 107 for introduction of a gasification agent 109 such as steam and of the hot particles 102 separated in the separator 104 through a downcomer 104a and for take-out of resultant gas 106 through gasification of raw material M in the fluidized bed 105, using the particles 102 as heat source.
The fluidized bed gasification furnace 107 in FIG. 3 comprises an introduction portion 107a for introduction of the hot particles 102 from the separator 104, a gasification portion 107b for introduction and gasification of raw material M, a lower communicating portion 108 for communication between the portions 107a and 107b at a lower portion in the fluidized bed 105 for allowing movement of the particles 102, and a gasification agent box 110 extending below the portions 107a, 107b and 108 for supply of a gasification agent 109 such as steam. The lower communicating portion 108 provided in the fluidized bed 105 provides a backflow preventive structure for preventing the fluid gas in the fluidized bed combustion furnace 100 from flowing back into the separator 104.
Arranged between the gasification portion 107b and the fluidized bed combustion furnace 100 is particle supply means 111 comprising an L-shaped portion 111a connected at its upper end to an upper layer portion of the fluidized bed 105 in the gasification portion 107b and a riser portion 111b rising again from a lower end of the L-shaped portion 111a and connected to a lower portion of the fluidized bed combustion furnace 100, thus providing a backflow preventive structure for preventing the fluid gas in the fluidized bed combustion furnace 100 from flowing back into the gasification portion 107b. In FIG. 3, reference numeral 10a denotes supplementary fuel supplied to the fluidized bed combustion furnace 100 as needs demand.
In the circulating fluidized bed furnace as shown in FIG. 3, it is required to enhance gasification efficiency of the raw material M in the fluidized bed gasification furnace 107 by increasing a circulatory amount of particles 102 between the furnaces 107 and 100 and to increase a production amount of resultant gas 106 by increasing a gasification throughput of the raw material M.                [Reference 1] JP 2005-274015A        [Reference 2] JP 2004-132621A        [Reference 3] JP 2005-41959A        